L. F. Spath — Notes on Ammonites. 65 



IV. — Notes on Ammonites. 



By L. F. Spath, B.Sc, F.G.S. 



(Continued from p. 35.) 



II. 



Obliquity of the Sdtuke-line. 



A CHARACTER of the suture-line that has received considerable 

 J\_ attention lately is the obliquity with regard to the radius. Of 

 course, it had long been noticed that suture-lines may vary in the 

 form and foliation of their elements (brachyphyllic, dolichophyllic, 

 and leptophyllic suture-Ikies of Mojsisovies, 1 and euryphyllian and 

 stenophyllian suture-lines of Haug 2 ) as in their general course. 

 There maybe (externally) a strong convexity forward (Cyclolobus), 

 a straight (Sphenodiscics) or wavy line (Pseudosageceras), or a con- 

 vexity backward (Protengonoceras). Again, the suture-line may be 

 inclined strongly forward towards the umbilicus (Cheltonia) or have 

 a retracted or dependent inner portion (Psiloceras). It is this latter 

 obliquity that has been used as a generic and even family distinction, 

 e.g. by Mr. Buckman, 3 to determine the affinity of Bredya with 

 Hanimatoceratidse, and not Hildoceratidse. 



On a previous occasion 4 when dealing with the "suspensive 

 lobe" (dependent auxiliaries) of Psiloceras and its ancestors, the 

 writer expressed the opinion that its significance was doubtful. 

 Since then the dissection of a number of Ammonites showing this 

 obliquity of the suture-line towards the umbilicus, e.g. Derocerates, 

 and the developing of the whole of their external and internal 

 suture-lines has confirmed his belief in the impossibility of using 

 this variable character — like the above-mentioned divisions proposed 

 by Mojsisovics and Haug — even for generic distinctions. 



Mr. Buckman, in his Monograph of the Inferior Oolite Ammonites, 5 

 figures on pi. A, fig. 29, the suture-line of a Htldocems that has 

 a strongly ascending inner portion as compared with the type given 

 in fig. 28. Waehner 6 has shown that dependent auxiliaries are 

 neither always present in Psiloceras, nor always absent in Arietites. 

 Tornquist 7 figures suture-lines of Pictonice that show the typical 

 descent towards the umbilicus and others that are straight. And it 

 may be recalled here what R. Douville 8 says concerning the genus 

 Ifacrocephalites : "As regards the more or less great obliquity of the 



1 " Die Cephal. d. Hallstiitter Kalke " : Abh. k.k. Beiehsanst., vol. vi, p. 2, 

 1873-93. 



3 " Les Amm. du Permien et du Trias " : Bull. Soc. Geol. France, ser. Ill, 

 vol. xxii, p. 409, 1894. 



a "Certain Jurassic (Lias-Oolite) Strata of South Dorset; and their 

 Correlation " : Q.J.G.S., vol. lxvi, pp. 97-8, 1910. 



4 " Development of Tragophylloceras Loscombi " : Q.J.G.S., p. 352, 1914. 



5 Pal. Soc, vol. i, 1887-1907. 



6 '.'. Beitr. Kenntn. Tief. Zonen Unt. Lias Nordostl. Alpen " : Beitr. Geol. 

 Pal. Osterr.-Ung., vol. iv, pts. iii, iv, pp. 190-202. 



7 "Die Degenerierten Perisphinctiden des Kimmeridge von Le Havre": 

 Abh. Schweiz. Pal. Ges., vol. xxiii, p. 41. 



8 " Etude sur les Cardioceratides de Dives, etc." : Mem. No. 45 Soc. Geol. 

 France, Pal. i, 19, fasc. ii, p. 14. 



DECADE VI. — VOL. VI. — NO. II. 5 



